Recently, it has been required to reduce the size of multilayer ceramic capacitors while enlarging their capacitance. To meet this requirement, the thickness of the dielectric layers must be reduced by using a raw material powder with a small particle size.
The capacitance of multilayer ceramic capacitors is measured with an AC voltage, not a DC voltage. However, a DC voltage is inevitably applied in actual use, and this application of a DC voltage causes a decrease in the capacitance of most capacitors. The decreased rate of the capacitance caused by the application of a DC voltage is referred to as DC bias characteristics, and there is a demand for capacitors having good DC bias characteristics.
Prior art multilayer ceramic capacitors are manufactured as follows.
First, barium titanate powder, which is the main component, and powders of metal oxides, which are sub-components and micro-additives, are mixed with a dispersion medium of water and a mixing medium of zirconia balls, to form a slurry in which barium titanate and the metal oxides are homogeneously dispersed.
Next, the slurry has the zirconia balls removed and is dried, to obtain a powder mixture.
Subsequently, this powder mixture is mixed with organic substances such as a binder and a plasticizer, to produce ceramic green sheets.
Thereafter, the ceramic green sheets and internal electrodes are alternately layered, and the laminate is sintered. Lastly, external electrodes are formed, to obtain a multilayer ceramic capacitor.
As the prior art pertinent to this kind of multilayer ceramic capacitor, for example, Japanese Laid-Open Patent Publication No. 2002-226263 is known.
Regarding the above-mentioned manufacturing method, this prior art proposes providing grain-growth-inhibiting particles at the grain boundary of crystal grains of barium titanate, to obtain good DC bias characteristics. This proposal uses, for example, zirconia balls with a diameter of 3 mm as the mixing medium when barium-zirconium titanate powder having a mean particle size of 0.3 μm is mixed with additives. However, since these zirconia balls are large in both diameter and mass, an excessive force is applied to the barium-zirconium titanate powder and the additives upon mixing, thereby resulting in excessive grinding.
If the barium-zirconium titanate powder or the barium titanate powder, which is the main component, is excessively ground, the particle size of the powder varies greatly. In addition, the excessively ground small particles facilitate grain growth during sintering, so that the particle size of the resultant crystal particles becomes large.
When a DC voltage is applied to a multilayer ceramic capacitor including such large crystal particles, a problem of deterioration of the DC bias characteristics arises.
Accordingly, by suppressing the variation in crystal particles, the present invention aims to provide a multilayer ceramic capacitor having good DC bias characteristics.